Mass spectrometers enable precise determinations of the constituents of a material. There are several distinctly different types of mass spectrometers. They all provide separations of all the different masses in a sample according to its mass to charge ratio. The molecules of the sample are disassociated/fragmented into charged atoms or groups of atoms, i.e. ions, and the ions are introduced into a region where they are acted upon by magnetic or electric fields which can be manipulated to separate the ions because the forces on the ions depend upon their mass to charge ratio.
The quadrupole mass spectrometer is one form of spectrometer device which does not employ magnets but utilizes radio frequency and/or DC fields in conjunction with a specifically shaped electrode structure. Inside the structure, the RF fields are shaped so that they can interact with certain ions causing a restoring force to induce such ions to oscillate about an electrically neutral position. A form of the quadrupole known as the quadrupole ion trap (QIT) has become important in recent years as a result of the development of more convenient techniques for handling the ions. The QIT device enables restoring forces in all three directions and can actually trap ions of selected mass/charge ratio inside the structure. The ions so trapped are capable of being retained for long periods of time which enables and supports various experiments which are not convenient in other apparatus.
In the use of a QIT, ions are usually confined by the RF field and then sequentially ejected to a detector by either ramping the RF trapping field voltage applied to the ring electrode or by applying a supplemental secular resonance frequency excitation to the end caps or applying a scan and a supplemental field simultaneously.
Another application of the QIT is in the so called MS/MS mode where a range of masses are trapped; mass scanning and/or resonance ejection employed to confine particularly chosen ions; then, disassociating the parent ions by collisions and separating/ejecting the fragments and obtaining a mass spectrum of the daughter ions.
When ejecting ions from the trap to the detector, in most prior art apparatus, equal percentages of ions were ejected toward both end caps. Since the ion detector was installed in only one end cap, the sensitivity was not maximized.
In U.S. Pat. No. 4,882,484, an apparatus and technique is disclosed and described for compressing the path of oscillations of ions in a trap so that the ions which impact the end cap are focussed toward the center of the end cap. This patent claimed a significant sensitivity improvement. This '484 patent also recognized that it would be beneficial to impact the ions on the correct end cap containing the detector. To accomplish this result, it is proposed to introduce a third order field non-linearity by shaping the ring and the end caps or to apply a small static DC voltage between the end caps. This '484 patent also describes static superimposition of higher order field distortions made possible by changing the shapes of the electrodes from a pure hyperbolic. German Patent No. DE4017264A1 and the journal article at Int. J. Mass Spectroscopy and Ion Process, Vol. 106, 1991, p. 63-78, also describe superimposition of multipole fields as a means to improve sensitivity.
The creation of special complicated surfaces as described by DE4017264A1 and U.S. Pat. No. 4,882,484 is very expensive and difficult. Also, due to the requirements for non-linear resonance, only certain selected ejection excitation frequencies are possible, such as 1/3 RF trapping field frequency in a hexapole field. Another disadvantage is that the relative magnitude of the quadrupole and hexapole or octapole field is fixed for a given set of shaped electrodes. The use of a small DC bias voltage applied to the end caps provides a superimposed static dipole field across the QIT. For small values of DC bias, no significant preferential effect in intensity is seen. For larger values of DC bias, intensity of larger mass ions is reduced. In addition, the application of a DC dipole field will cause the mass calibration curve for the trap to become nonlinear.